In warm coastal waters around the world, swimmers can often spot large groups of jellyfish pulsing rhythmically on the seafloor. Unless properly prepared with protective clothing, it is best to steer clear of areas that Cassiopea, or upside-down jellyfish inhabit: getting too close can lead to irritating stings, even without direct contact.
Click here for original story, Stinging water mystery solved: Jellyfish can sting swimmers, prey with ‘mucus grenades’
Source: Phys.org